<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
         #box{
             /* width: 100px;
             height:100px; */
             border-bottom: 50px solid transparent;
             border-top: 50px solid transparent;
             border-right: 50px solid rgb(231, 64, 64);
         }

    </style>
</head>
<body>
     <div id="box">

     </div>
</body>
</html>